Luminescent (e.g. fluorescent) optical sensors are widely used in medicine, food processing and HVAC systems for determining the concentration of different gases such as oxygen, carbon dioxide and others. Examples of conventional fluorescent optical sensors are disclosed in U.S. Pat. No. 6,682,935 and U.S. Pat. No. 5,728,422.
FIG. 1 depicts a representative example of a conventional sensor. A radiation source 1 such as a light-emitting diode illuminates a gas-sensitive, photoluminescent layer 2, which here is supported on a backing film. In response to at least one wavelength emitted by the source 1, the layer 2 luminesces, emitting radiation at a different wavelength. The intensity of emitted luminescence depends not only on the intensity of incident radiation from the source 1 but also on the concentration of a target gas in the vicinity of the layer 2. A photo detector 3 receives luminescence emitted by layer 2 inside a spatial angle 4. The detector 3 is responsive to the emitted luminescence wavelength and outputs a measurement signal related to the detected intensity, and hence to the concentration of the target gas. Typically a focusing system 5 is provided to focus the radiation onto detector 3 via an optical filter 6.
The light-emitting diode (LED) or other radiation source 1 emits electromagnetic radiation across a waveband including specific wavelengths of which one or more will be absorbed by the gas sensitive layer 2 and give rise to photoluminescence. Typically the emitted luminescence will have a longer wavelength than those absorbed by the layer. However, the luminescent wavelength may be close to the waveband emitted by the LED or could even fall within the waveband, and as such the waveband to which the detector 3 is responsive will typically overlap that of the LED. As shown in FIG. 1, some of the radiation from the LED 1 passes though the sensitive layer 2 to the detector 3 and hence it is necessary to provide an optical filter 6 in order to suppress the high intensity LED emission and allow the luminescence to pass through, so as to distinguish the luminescence from the LED radiation. The degree of suppression achieved by the filter 6 must be high because generally the luminescence is relatively weak. In particular, in order to increase the efficiency of the sensor, the radiation source 1 often incorporates its own focusing system (not shown), which concentrates the LED radiation onto a spot on the sensitive layer 2, leading to a difference between the intensities of the LED radiation and the luminescence which could be more than four or five orders of magnitude. This means that the filter 6 must be a high quality optical filter, such as an interference filter for instance. Filters of this sort are complex to manufacture and expensive.
In some conventional sensors, the optical scheme is modified from the linear arrangement depicted in FIG. 1 to a more compact scheme in which both the radiation source 1 and detector 3 are placed on the same side of the gas sensitive layer 2. FIG. 2 shows an example, using like reference numerals for like components already described in relation to FIG. 1. In certain regards, a non-linear optical scheme such as this has better protection from direct LED radiation exposure, because at least no straight-forward light from the LED is received at detector 3. However, the required degree of LED radiation suppression is still high since there is a lot of scattered light and hence it is still necessary to provide a high quality optical filter 6.